Mixers are used in communications circuits for the purpose of generating a modulated carrier for transmission, demodulating a modulated carrier in reception, or converting a signal at some input intermediate frequency (IF) to another output radio frequency (RF) by multiplying two input signals and thereby generating a third. A number of mixer realizations, both passive and active, are known in the art, and double-balanced mixers are known particularly well due to their advantages in the suppression of unwanted spurious signals and the isolation of any one of three ports to the other two, there generally being two inputs and one output. The Gilbert Cell has been the most widely used active mixer circuit for performing the above tasks, usually incorporated within an integrated circuit. It does, however, possess certain limitations in terms of intermodulation (IM) distortion and noise figure (NF) that precludes it""s use in communications systems having demanding performance specifications. The series-shunt feedback mixer delivers a much improved IM performance, but the lossy nature of the feedback topology does not improve the NF performance.
Referring to FIG. 1, a schematic diagram of a series-shunt feedback mixer is shown in a form that delivers exceptional overall performance. Here, the mixer is comprised of switching transistors 101, 102, 104, and 105, which are turned on (saturation) and off (cutoff) alternately by a differentially applied local oscillator (LO) signal. By this switching action, a pair of currents generated by amplifying transistors 103 and 106 are divided into four paths, there being two paths for each of two currents. The currents generated by switching transistors 103 and 106 are the result of an input intermediate frequency (IF) signal applied differentially across their respective base connections and the series feedback resistors 110 and 111. The current source 112 serves to establish the quiescent bias condition of the mixer. The hybrid transformers 115 and 116 combine the four currents from switching transistors 101, 102, 104, and 105, creating a pair of feedback voltages 121 and 122, as well as an output RF signal 123. The shunt feedback resistors 107 and 108, in conjunction with the series feedback resistors 110 and 111 and the amplifying transistors 103 and 106, form a pair of series-shunt feedback amplifiers which serve to establish the conversion gain and improve the IM performance of the mixer.
Those familiar with the art will readily understand that the NF performance of the series-shunt feedback mixer is impaired by the dissipative, or lossy, nature of the feedback topology. This active mixer does offer considerable advantages over the more traditional Gilbert Cell active mixer, especially in terms of signal-handling and performance variations over temperature due to the temperature dependency of the emitter resistance re, and the tradeoffs that are encountered in receiver and transmitter system design. It has long been desirable that a mixer, either passive or active, be available that has improved IM and temperature performance, and at the same time has an improved NF performance without the expense of added power or complexity.
It is the purpose of this invention to further advance the art of feedback mixers by addressing the sources of noise present in the series-shunt feedback mixer, and to therefore provide an active mixer of markedly improved NF performance, while at the same time conserving power consumption and retaining the IM performance and overall sense of simplicity and cost effectiveness of the series-shunt feedback mixer.
A lossless feedback double-balanced active mixer circuit with improved intermodulation (IM) and noise figure (NF) performance is described which includes a pair of lossless feedback balanced active mixer circuits, each of which includes a differential pair of switching transistors which divide a controlled current into two paths at a rate determined by an input local oscillator (LO). A hybrid transformer in each lossless feedback balanced mixer, consisting of a centre-tapped primary winding and a secondary winding, combines the two currents to provide a recombined amplified IF signal and an output radio frequency (RF) signal. A third amplifying transistor in each lossless feedback active mixer circuit provides the controlled current, which is determined by an input intermediate frequency (IF) signal. Each lossless feedback active mixer circuit further includes a feedback transformer, comprised of an input winding and a tapped output winding, which compares the input IF signal with the recombined amplified IF signal from the hybrid transformers and applies the difference as a correction to the amplifying transistors, thereby completing a lossless feedback amplifier circuit and in turn improving the IM performance of the mixer circuit. Since the feedback transformer is essentially lossless, it introduces no significant sources of noise to the active mixer circuit, and therefore the NF of the of the lossless feedback active mixer circuit remains unimpaired beyond the NF of the transistors themselves. An additional pair of complementary amplifying transistors may be added to improve the IM performance still further. The connection of the secondary windings of the hybrid transformers of the lossless feedback active mixer circuits effectively cancels the output LO and IF signals and provides an output RF signal.